Optical projection systems typically include a transmissive or a reflective imager, also referred to as a light valve or light valve array, which imposes an image on a light beam. Reflective light valves modulate only selected portions of the input beam to form an image, and provide important advantages over transmissive light valves. For example, reflective light valves permit controlling circuitry to be placed behind the reflective surface, and more advanced integrated circuit technology is available because the substrate materials are not limited by their opaqueness.
Many reflective liquid crystal display (LCD) imagers rotate the polarization of incident light. Thus, polarized light is either reflected by the imager with its polarization state substantially unmodified, or with a degree of polarization rotation imparted to provide a desired grey scale. Accordingly, a polarized light beam is generally used as the input beam for reflective LCD imagers. A desirable compact arrangement includes a folded light path between a polarizing beamsplitter (PBS) and the imager, wherein the illuminating beam and the projected image reflected from the imager share the same physical space between the PBS and the imager. The PBS separates the incoming light from the polarization-rotated image light. A single imager may be used for forming a monochromatic image or may also be used for forming a color image when the single imager is illuminated with light whose color is temporally varied. Multiple imagers may be used for forming a color image, where the illuminating light is split into multiple beams of different color, for example red, green and blue. An image is imposed on each of the colored beams individually by respective imagers, and the differently colored image beams are then recombined to form a full color image beam.
It is desirable to use as much light generated by the light source as possible. Where the light source generates light over a wide angle, such as an arc lamp, more light can be passed through the imager system using low f-number optics. A problem, termed “polarization cascade” and associated with a conventional PBS that relies on Brewster effects to polarize the light, places a lower limit on the f-number of the illumination optics of traditional optical imaging systems. A conventional PBS used in a projector system, sometimes referred to as a MacNeille PBS, uses a stack of inorganic dielectric films placed at Brewster's angle. Light having s-polarization is reflected, while light in the p-polarization state is transmitted through the polarizer. However, wide angle performance is difficult to achieve using these polarizers, since the Brewster angle condition for a pair of materials is strictly met at only one angle of incidence. As the angle of incidence deviates from Brewster's angle, a spectrally non-uniform leak develops. Furthermore, there are contrast disadvantages for a folded light path projector associated with the use of p- and s-polarization.
Since light in a projection system is generally projected as a cone, most of the rays of light are not perfectly incident on the polarizer at Brewster's angle, resulting in depolarization of the light beam. The amount of depolarization increases as the system f-number decreases, and is magnified in subsequent reflections from color selective films, for example as might be found in a color-separating prism. It is recognized that the problem of depolarization cascade effectively limits the f-number of the projection system, thereby limiting the light throughput efficiency.